Reactor leak detector using filters



April 13, 1965 I. M. JACOBS 3,178,355

REACTOR LEAK DETECTOR USING FILTERS Original Filed Aug. 13, 1958 3Sheets-Sheet l HEA T SINK FUEL REGION 0!? CHAN/VII- April 13, 1965 I. M.JACOBS REACTOR LEAK DETECTOR USING FILTERS 5 Sheets-Sheet 2 OriginalFiled Aug. 13, 1958 FUEL CHANNELS 34 R00 ASSEMBLY FUEL 4 OOOO FILTERVESSEL I42 .ILJLJLJ 1 FUEL Cl-MA/A/ELS 94 Fig.9.

INVENTOR. [VAN M. JAcoss,

BY A TTOR/VEY A ril 13, 1965 l. M. JACOBS REACTOR LEAK DETECTOR USINGFILTERS 3 Sheets-Sheet 3 Original. Filed Aug. 13, 1958 INVENTOR. IVAN M.JACOBS,

ATTORNEY United States Patent Oil" 3,178,355 Patented Apr. 13, 19653,178,355 REACTOR LEAK DETECTOR USWG FILTERS Ivan M. Jacobs, San Jose,Calif., assignor to General Electric Company, a corporation of New YorkOriginal application Aug. 13, 1958, Ser. N 0. 754,861, now Patent No.3,069,339, dated Dec. 18, 1962. Divided and this application Oct. 9,1961, Ser. No. M39413 9 Claims. (6]. 176-19) This application is adivision of my copending application Serial No. 754,861, now US. Patent3,069,339, filed August 13, 1958, and entitled Nuclear Fuel Element LeakDetector.

This invention relates to the liberation of energy in nuclear reactors,and it relates more particularly to an improved method and apparatus forthe detection of the existence and identity of leaking fuel elements insuch reactors, as in high neutron flux test and power reactors.

The release of large amounts of energy through nuclear fission reactionsis now quite Well known. In general, a fissionable atom such as U U orPu absorbs a neutron in its nucleus and undergoes a nucleardisintegration. This produces on the average, two fission products oflower atomic weight and great kinetic energy, and from 2 to 3 neutronsalso of high energy. For example, the fission of U produces a lightfission product and a heavy fission product with mass numbers rangingbetween 80 and 110 and between 125 and 155 respectively, and an averageof 2.5 neutrons. The energy release approaches about 200 mev. (millionelectron volts) per fission.

The kinetic energy of the fission products is quickly dissipated inambient material as heat. If after this heat generation there is atleast one net neutron remaining which induces a subsequent fission, thefission reaction becomes self-sustaining and the heat generation iscontinuous. The heat is removed by recirculating a coolant through heatexchange relationship with the fissionable material and a heat sink. Thereaction may be continued as long as suflicient fissionable materialremains in the system, considering the effects of the fission productswhich also may be present.

In order to maintain such fission reactions at a rate energy, nuclearreactors are presently being designed, constructed, and operated inwhich the fissionable material or nuclear fuel is contained in fuelelements which may have various shapes, such as plates, tubes, or rods.These fuel elements are usually provided with a corrosion resistantnon-reactive cladding on their external surfaces and which contains nofissionable or fertile material. The elements are grouped together atfixed distances from each other in a coolant flow channel or region as afuel assembly, and sufficient fuel assemblies are combined to form thenuclear reactor core capable of the self-sustained fission reactionreferred to above. The core is enclosed within a reactor vessel throughwhich a coolant is circulated.

The cladding serves two primary purposes; first, to resist any chemicalreaction between the nuclear fuel and either the coolant or moderator ifpresent, and second, to prevent the highly radioactive fission productsfrom being released into the coolant or moderator or both. Commoncladding materials are stainless steel, aluminum and its alloys,zirconium and its alloys, and others. The failure of the cladding cancontaminate the coolant or moderator and the coolant system withintensely radioactive long-lived products to a degree which interfereswith plant operation. Detection and replacement of the defective fuelelement are thus desirable before a major clad rupture and suchcontamination occur.

Conventionally, the gross activity of the reactor coolant or the off gasin the system is monitored during operation, and this readily indicatesby an abrupt activity rise the occurrence of a cladding leak somewherein the reactor core. In such case the reactor is normally shut downbefore excessive coolant contamination occurs. The gross activitymonitoring system does not, however, indicate which one or more of theperhaps many hundreds of fuel assemblies are defective and leaking.

The identifiction of the particular fuel element or elements which aredefective has been accomplished in the past by continuously orintermittently sampling the effluent coolant from each fuel assembly orchannel in the reactor, or from groups of such assemblies, andcontinuously or intermittently monitoring the radioactivity of eachindividual sample in a sample room'located away from the reactor. Thisprocedure, however, requires either the penetration of the reactorvessel by a large number of sample lines, or the use of valve manifoldsor mechanically complex remotely-operated multi-p ort valves locatedinside the vessel, and which successively sample the efiluent coolantfrom the various channels.

It is accordingly, a primary object of this invention to overcome theforegoing disadvantages and provide an improved process and apparatusfor detection of the existence and identity of defective and leakingfuel elements in a nuclear reactor core.

It is a further object to provide for the monitoring of' theradioactivity of the effiuent cool-ant from each fuel assembly orchannel in a nuclear reactor without plural penetrations of the reactorvessel and without complex valving.

An additional object is to provide a leaking fuel element detectionsystem which effectively integrates the leakage of radioactive materialsfrom the defective fuel element over a period of time and enhances theefiiciency of detection.

Other objects and advantages of this invention will be come apparent tothose skilled in the art as the description and illustration thereofproceed.

Briefly, the present invention comprises an improved process andapparatus for detecting the existence of and identifying defective andleaking fuel elements in nuclear reactors including the steps of andmeans for withdrawing a sample of fluid flowing adjacent a given fuelelement and containing any radioactive fuel and fission producingparticles discharging from a defective fuel element, passing at leastpart of the sample through contact with a body of solid filter materialto filter at least part of the particulate material from the samplestream, and subsequently monitoring the partially spent filter materialto determine the presence thereon of radioactive materials. When thefluid is a boiling coolant, the sample may be separated into a vapor anda liquid phase, and the liquid. phase passed alone through the filter;or the mixed phases may be filtered. When the fluid is a non-boilingcoolant, that is, one which remains in the vapor or liquid phase in thenuclear system, a small fraction is separated from the main stream :andis passed through the filter. If no particulate radio-active materialsare leaking into the fluid, the filter reactivity will have a relativelylow background value, but it will be anomalously high when the fuelelement is defective and leakage is occurring. In large systems wherepluralfuel channels or regions are employed, plural samples are taken,one from each channel, an individual filter element is used to treateach sample, and

an anomalously radioactive filter element indicates the existence of andidentifies the channel or region containing the leaking fuel element asthe one from which the sample contacting that filter was taken. In thisway even minute leaks are located prior to the time they become majorruptures.

After reactor shutdown, due to a fuel element leak as may be shown by agross coolant activity rise, or in the normal course of refueling, theindividual filter elements are removed to a monitoring station. Ifanomalously high activity is detected on a given filter element, it willindicate the channel, group of channels, or region in which the leak orleaks have occurred. Replacement fuel elements or assemblies are thusquickly and efiiciently substituted for the defective ones.

Although a major rupture may occasionally occur and will etIectarelatively large increase in gross coolant radioactivity indicating thenecessity of an immediate reactor shutdown, more commonly the claddingleaks will be tiny fissures through which radioactive particles leakslowly and will not be sufiicient to require an immediate shutdown.Leakage of these materials from such fissures continues at varyingrates, and due to thermal cycling of the reactor during normal operationthese fissures may gradually open up until ultimately a gross escape ofradioactive fuel and fission products from a major rupture could occur.

The collection of radioactive particulate matter according to thisinvention permits detection and identification of leaking fuelassemblies while the leaks are still in this fissure stage, and beforegrossescape occurs. is due in part, to the integrating effect ofcollecting, over an extended operating period, the radioactive materialswhich may be escaping at a very low rate through the tiny fissures inthe clad. This rate is frequently so low that it cannot be detected asgross activity in the coolant stream. By removal of the filter elementsfrom the reactor at each scheduled shutdown and by monitoring eachelement, the existence of these ruptures can be detected well in advanceof the time they would open up and become serious. The defective fuelassembly is replaced with a new one and operation continues. Thisprocedure reduces significantly the number of unscheduled shutdowns dueto major breaks in fuel elements.

The present invention, including the process operation and severalembodiments of the invention in specific apparatus structure, will bedescribed in greater detail and will be more readily understood by thoseskilled in the art through reference to the accompanying drawings inwhich:

FIGURE 1 is a schematic block diagram showing the general operations ofthe process of this invention,

FIGURE 2 is a schematic diagram showing the present invention applied toa nuclear reactor having several fuel channels through which a coolantis recirculated,

FIGURE 3 is an elevation view in partial cross section of a typicalnuclear reactor vessel showing the location of the coolant samplingpoints and the filter element holder vessel above or apart from thereactor core flow channels,

FIGURE 4 is a split plan view of FIGURE 3 showing in the northeastquadrant the top of the reactor core with its fuel element assembliesand flow channels, control rods, and fuel elements, and showing in thenorthwest quadrant the top of the sampling grid and the location of oneof the four filter holder vessels,

FIGURE 5 is an elevation view of FIGURE 4 showing the upper end of thereactor core, the sampling grid, and the attachment means for securingthe lower ends of the sample lines in the sampling grid,

FIGURE 6 is an enlarged detail view in partial cross section showing thedetail of the filter holder vessel indicated in FIGURES 3 and 4 andcontaining a removable filter element holder and the individual filterelements,

FIGURE 7 is a plan view of the filter holder vessel shown in FIGURE 6,

FIGURE 8 is an elevation view in cross section of a typical individualfilter element suitable for use in the reactor detailed in FIGURES 3-7,

FIGURE 9 is an elevation view in cross section of another embodiment ofthis invention in which the individual filter elements are disposeddirectly in the sampling grid,

This

FIGURES 10 and 11 are views of two other kinds of filter elementssuitable for use in various kinds of coolants,

FIGURES 12 and 13 are views of still another embodiment of thisinvention in which flow of coolant through the filter element is inducedby the coolant flow through the individual fuel channels in the reactorcore, and

FIGURE 14 shows the application of a shielded and focusedspectrophotometer to the individual monitoring of fuel elements in agrid.

Referring now more particularly to FIGURE 1, a schematic diagram showingthe process steps of this invention is presented. The bounds of thereactor vessel are indicated at 10 and contains fuel channel or fuelregion 12. Coolant enters and leaves the vessel 10 by means of lines 14and 16. The heated coolant is circulated through heat sink 18 where theheat is removed. The heat sink may be a heat exchanger, a turbine andcondenser system, or both, or other known heat using device. A sample ofthe coolant is taken adjacent the effluent point from fuel region 12 bymeans of line 20 and passed into filter element holder 22. The filteredcoolant may be returned by line 24 to any convenient place in thecoolant cycle. The particulate material, containing any radioactive fuelor fission product particles, is retained on the filter element whilethe filtrate flows on through line 24.

The filter element is moved from filter zone 22 to monitor zone 26 whichincludes radioactivity sensingcounter-detector-amplifier-recorderequipment. The function here is to detect and record the radioactivitiesof individual filter elements in groups or in sequence and distinguishthose which have anomalously high activities. This serves to identifyand locate a defective fuel channel or region. It should be understoodthat FIGURE 1 merely illustrates the process as applied to a single fuelassembly or fuel region through which a stream of coolant flows, andthat in actual practice the many hundreds of individual fuel assembliesor fuel regions in a large nuclear reactor are each similarly sampledand monitored. The monitoring by measurement of the radiation from thefilter element may be done a considerable time after reactor shutdown.

FIGURE 2 is another schematic diagram showing the general physicalrelationship of some of the various zones referred to in FIGURE 1.Nuclear reactor vessel 40 containing reactor core 42 is illustrated ascontaining three representative fuel channels or regions a, b, and c.The core is provided with coolant inlet and outlet headers 44 and 46connected respectively with inlet and outlet lines 48 and 50,communicating with suitable coolant circulation and heat sink equipmentdesignated generally as 52. The reactor core is further provided withcontrol rods 54 positioned by control rod drive mechanism 56, the rodsbeing extendable into and out of the core.

The coolant samples are taken just upstream from the coolant outletopening of each fuel channel and prior to gross mixing of the channelefiluents with each other in outlet header and line 46 and 50respectively. The sample from channel a for example, passes throughsample line 58 through a communicating sample filter chamber 60containing filter element 62. The filtrates from all such channelsdischarge into a common sample header or manifold 64 from which thefluid is vented through line 66 controlled by valve 68 to any convenientregion of lower pressure, such as part of the coolant system, acondenser, a blower or pump not shown, or the like. Purge line 70controlled by valve 72 opens into line 66 and is used to reverse flushthe sample lines and filter elements clear when necessary.

Gross coolant monitor-controller 74 is provided, connected to beresponsive toa predetermined rise or level in gross coolantradioactivity. Several modes of operation are permitted.

In one mode, valve 72 is normally open and a controlled back flow ofclean coolant fluid or fluid compatible with the coolant, is introducedin the reverse direction through the sampling system preventing samplefiltration. Valve 68 is normally closed. Upon detection of apredetermined level or rise in gross coolant radioactivity, controller74 closes valve 72 stopping the back fiow and opens valve 68 permittingsampling to begin. Sample filtration continues for a short periodsufficient to filter out particles liberated from the defective fuelchannel or channels into the coolant and partially saturate the filterelement in the defective channel. In cases of severe or multipleruptures, this period may be equal to or less than the time necessaryfor the gross coolant to flow from the detection point 76 through thecoolant recirculation system to the sampling points. Longer periods maybe used with less serious ruptures. At the end of the sampling period,controller 74 activates control rod drive mechanism 56 driving rods 54into the core shutting down the reactor. The filter elements 62 are thenremoved and monitored in any convenient way as hereinbeloW-described toidentify the defective channel.

In another mode of operation, valve 72 is normally closed and valve 68is normally open sufiiciently to permit a small controlled flow ofcoolant sample to be taken through each filter from its correspondingfuel channel. At a regularly scheduled reactor shutdown, as forrefueling, controller 74 closes valve 68 and shuts the reactor down asbefore. If desired, by opening valve 72 'a very small back flow of cleancoolant may be admitted at a rate insuflicient to dislodge particlesfrom the filters, but sufficient to prevent diffusion of contaminatedgross coolant into contact with the filter elements which mightadversely affect the signal to background ratio. The filter elements aremonitored either after removal from the reactor or in place, and ananomalously high filter activity identifies the defective and leakingfuel channels.

In either case, the filter elements 62 are selected upon a considerationof their physical characteristics, the sample flow rates, and thefiltration time so that they do not become fully saturated withparticulate material from the samples treated. In other words, thesample filtration system is constructed and operated so that uponreactor shutdown and removal of the filter elements from the reactor,even those filters which treat coolant samples from the defective fuelchannels are not fully saturated and still retain excess filtrationcapacity.

Clearly from the foregoing description of FIGURE 2, care is taken tomaintain a substantial signal to background ratio in the system byavoiding operation following an indicated fuel channel rupture for aperiod long enough to saturate any or all the filters with contaminatedcoolant for example.

The interference or background signal, normally encountered due to thepresence of activated corrosion or erosion products or of fuel andfission products circulat- 7 ing with the coolant, is minimized oreliminated by continuous purification of the gross coolant flow. In thecase of water cooled reactors, this is done by coolant demineralization,filtration, or both. In reactors cooled with liquid metals, the usualmetal purification techniques are used. In reactors cooled with organicliquids, such as terphenyl and the like, techniques such as filtrationor distillation or both may be used. In gas cooled reactors, the gascoolant may be similarly purified by filtration, settling, centrifugalseparation, or electrostatic precipitation. Other purificationprocedures will occur to those skilled in the art and are effected inzone 52 shown in FIGURE 2. Such procedure aids in further reduction ofthe background radioactivity signal, which is usually on the order of l/nth that generated by a filter sampling coolant from a leaking fuelchannel. This is due to the effect of uncontaminated coolant dischargingfrom n1 non-leaking channels and which mixes with and dilutes thecontaminated coolant discharging from one defective channel. Thisdilution occurs during passage of the gross coolant flow through thecoolant path connecting the reactor coolant outlet with its inlet.Although a greater numberof defective channels will tend to bring thesignal to background ratio down, the con-.

tinuous coolant purification procedures tend to raise this ratio, andthus counteracts the effect.

Referring now to FIGURE 3, an elevation view in partial cross section ofa typical high pressure commercial scale nuclear power reactor vessel 81is shown provided with a removable upper head 82 and head flanges 84 and86. Coolant inlets 88 enter the vessel at the lower end, and coolantoutlets 90 are provided somewhat above the mid-height of the vessel. Thereactor core 92, consisting of a plurality of vertically disposed,parallel, fuel assembly and flow channels or regions 94, is supported inthe vessel by means of support 96. The channels or regions are open attop and bottom to permit coolant flow therethrough, and each channelsurrounds or includes a bundle of individual nuclear fuel elements asshown in FIGURE 4. A typical control rod is indicated generally at 98and extends into the core to permit variation in the rate of heatgeneration.

Disposed peripherally around the inner wall of vessel 80 is a supportmeans 100, here shown as a ring of L-shaped cross section forming asupport shelf. Sup ported on this shelf by means of support element 102is a removable structural cage-like assembly consisting of lower ring104, middle ring 106, and upper ring 108 spaced vertically apart fromone another and connected by vertical members 110 and 112. Suspendedfrom the lower ring 104 is an upwardly flaring coolant turning vane 114having lower open end 116 and serving to deflect heated coolant, flowingfrom core 92, laterally toward the several coolant outlet nozzles 90. Asshown in the drawing, the upper outer periphery of the turning vane liesjust above the outlet nozzles. A typical instrument line 118 for in-coreinstrumentation extends vertically from the top of core 92 to the toppart of the removable assembly, where it joins line 120 attached to andextending through vessel head 82. Suspended from lower ring 104 by meansof support member 122 is sampling and core hold-down grid 124 locatedjust above the upper surface of reactor core 92. The heated coolantpassed upwardly through sampling grid 124 and coolant samples are takencontinuously from the eflluent of each flow channel before theseefiluents mix with each other above the core. The plan views of both thereactor core 92 and grid 124 are shown in FIGURE 4 subsequentlydescribed.

From the various sampling points in grid 124-, sample lines 126 passupwardly into bundles 123, enclosed by sample line protector tube 130,and enter the filter element vessel 132. The structure of these vessels,their contents, their number, and their disposition through the vesselcross section are all shown more clearly in FIGURES 4 to 8, describedbelow.

Vent line 134 opens from each filter element vessel 132 through header136 and discharges to a low pressure region of the coolant recirculationsystem. Although this low pressure point may be in the reactor vesseldome itself, it is shown in FIGURE 3 external to the reactor vessel.Flow indicator 138, controller 146 and valve 142, and purge inlet 144and valve 146 areall provided for vent line 136. By means of controller146, depending upon the mode of detector operation, the sample flow iseither started upon a predetermined signal during reactor operation, oris maintained continuously and is controlled at a proper rate inrelation to the volume of the sampling and filtration system so thatsubstantially equal flow rates of coolant samples are maintained througheach individual filter element,=and so that at no time does a filtertreating the eflluent sample from a leaking fuel channel becomesaturated with radioactive particulate matter. It should be noted thatno complex valves are required, and that only 7 a single vent line 136penetrates reactor vessel 80 at point 148'.

In FIGURE 4 a split plan view is presented showing the northwestquadrant of the sampling grid 124 and the northeast quadrant of the topof the reactor core 92. Elements also shown in FIGURE 3 are heredesignated by the same numerals. In. the northeast quadrant are shownsquare fuel channels 94, each containing an assembly of fuel elements160, and control rod 98 having a cruciform crosssection operatingbetween four adjacent channels. In the northwest quadrant, the samplinggrid 124 is shown in detail. It is contained within a circular grid ring162 which serves as the end support for a system of east-west ribmembers 164 and north-south rib members 16*, intersecting at rightangles forming an egg-crate type grid. The spacing is such as to form aplurality of square sample openings or cells 168 corresponding to andaligned with the end of each of the fuel channels. Extending across thecenter of each sample cell 168 is a sampling tube support member 170which serves to locate the anchor 172 of the intake end of the sampleline, and any sample separator which may be required, in the center orany other selected position of the fuel channel effluent. The locationof one of the filter vessels 132 is shown together with its sample lineprotector tube 131).

In the large reactor system specifically illustrated here, there are 488fuel channels or regions. The equal number of samples which must befiltered through individual filter elements dictates that theypreferably be located as shown in FIGURES 3 and 4. Thus, four groups of122 filter elements each are disposed in individual holder vessels asshown, one in each quadrant of the reactor vessel cross section. Othermultiples of filter vessels may be used for larger or smaller numbers offuel regions or channels.

Referring now to FIGURE 5, which is an elevation view of FIGURE 4 takenas indicated, the top of the several fuel channels 94 and the samplinggrid 124 are shown. The intersecting members 164 and 166 of the grid aresupported at their ends by grid ring 162. The lower ends of the samplelines 126 extend down to anchor 172 which is provided with an opening174 therethrough as a part of the sample flow path. The anchor 172 issupported in each sample cell by support member 170. The alignmentbetween the upper end of each fuel channel and the opening or samplecell in the egg-crate sample grid is clearly shown.

Referring now to FIGURES 6 and 7, a detailed cross section elevationview of the filter element holder vessel 132 shown generally in FIGURE3, and a partial plan view, are shown respectively. The vessel 132 isformed of the central cylindrical section 189, a lower conicaltransition section 182 connected to the top of sample line protectortube 136 surrounding sample lines 126, and is provided with an upperremovable head 184 and lift ring 186. A holder support plate 138 islocated horizontally at the bottom of the vessel 132. It is perforatedin a pattern similar to that shown in FIGURE 7 to receive the upper endsof sample lines 126. The holder vessel is preferably made of a neutronabsorbing material, such asboron steel or the like, to minimize neutronactivation of the filter elements.

Supported within vessel 132 and upon plate 188 is the filter elementholder assembly consisting of lower plate 1%, upper plate 192, centralspindle 194, lift ring 196, and a plurality of filter element holdertubes 198 supported between the plates 190 and 192. These tubes, shownwithout filter elements in place, are arranged in a pattern aroundcentral spindle 196 as indicated in FIG- URE 7, in which upper plate192, the spindle, and the holder tubes 1% are also shown. Other patternsobviously can be substituted, if desired, to accommodate a greater orsmaller number of filter elements. Fluid outlet 2&6 is provided openingfrom the upper part of vessel 132 by means of which the filtrate portionof the samples taken is removed and conducted via line 134 shown inFIGURE 3 to a low pressure sink to maintain the controlled flow ofcoolant sample.

Referring to FIGURE 8, a detailed vertical cross section isshown of thestructure and contents of the filter element holder tube 198. Elementspreviously described are designated here by the same numbers. Filterelement support ring 2% is fitted'into the lower end of tube 198 andsurrounds the upper end of the sampling tube 126. Seal 22b2, such as anO-ring, is provided at the lower end of each tube 193 to prevent samplemixing. A fluid-permeable filter element 204, capable of retainingparticles as small as about 1.0 micron in average dimension is provided.It has a lower end fitting 266 tightly seated against ring 2119 by meansof loading spring 203, and an upper hold-down or retainer ring 210. Thefilter element in this example consists of a cylindrical fluid-permeablecup or tube closed at its upper end and fabricated of any suitablematerial such as stainless steel, aluminum, zirconium, nickel or othermaterial in a form which is fluid permeable, which has pores ofs'ufiiciently small size to retain the small escaping particles, andwhich suitably resists corrosion in the reactor environment. Packedglass wool makes an effective filter. Particularly suitable are thepermeable ceramics and sintered metal powders. Use of high-purityspectroscopic grade permeable graphite minimizes background radiationotherwise induced by neutron activation of the filter element whenplaced inside the reactor vessel as shown here. Specifically suchelements as cobalt, copper, manganese, and other readily activatedmaterials must be meticulously avoided for even as trace impurities inthe filter, neutron activation will result in an undesirably highbackground activity. The sample flow is through line 126 into theinterior of the filter element 204, through the permeable element wallto the annulus, and then the filtrate flows out through opening 212 inupper ring 210.

The modification of this invention described above and illustrated inFIGURES 3 through 8 is typical of its embodiment in a commercial scaleboiling light water moderated and cooled nuclear reactor powergeneration system. The following data are given as illustrative of thestructure and operation of such a system which liberates nuclear heat atthe rate of 685 megawatts, and which generates electrical energy at agross rate of 192 megawatts and at a net rate on the line of megawatts.

The nuclear reactor vessel is 13 feet in diameter and 42 feet high, thereactor core consists of 488 square zirconium channels 3.75 inches by3.75 inches on a side, 10 feet long, and containing 36 zirconium-clad U0fuel rods about 0.56 inch in outside diameterin a 6 x 6 array. The Uenrichment is 1.5%. The total weight of U0 contained in the reactor coreis about 66 tons. The coolantmoderator is light water circulated throughthe core at a rate of about 26x16 pounds perhour. The coolant ispartially vaporized producing about 1.5 X 10 pounds per hour ofsaturated steam at 1000 psi. The 488 sample line inlets are located inthe sampling grid which covers the top of the core, the sample lines aresegregated into four bundles of 122 lines each, and are connected intothe four filter element holder vessels as shown in FIG- URE 3. Thefilter elements are A1 0 sleeves as shown in FIGURE 8, are approximately3 inches long, inch inside diameter, /s inch in wall thickness.

In normal operation the reactor run is continued for periods up to 180days, at the end of which the reactor is shut down for routinemaintenance. The vessel is opened, the filter elements are removed formonitoring, and scheduled refueling is done. If any anomalousradioactivity is detected, an actual or incipient rupture exists, and anew fuel assembly is substituted for the defective one. Such anomalousactivity is that which is greater than the average of the other filterelement activities. The detection of radiation levels between fixedlimits, or the exclusion from detection of radiation levels above orbelow some fixed value, may be elfected with conventional radiationdetection instruments, amplifiers, and pulse analyzers which arecommercially available. In this manner signals from traces of corrosionproducts, coolant activities, and other materials do not interfere withthe desired signal from radioactive particles released from thedefective fuel element.

Referring now to FIGURE 9, another embodiment of this invention is shownin which the sample filters are placed in the sampling grid rather thanin a filter element holder vessel as in FIGURES 3, 4 and 6. Structuralelements shown in FIGURE 9 which are the same as in FIGURES 5 and 8 aredesignated by the same numerals. In FIGURE 9 sampling grid 124consisting of elements 164 and 166 includes filter holder tube 220supported from the sampling grid by a support spider including elements222. Filter element 204 is enclosed within the tube and sample flow isinduced positively through openings 224, through the filter element, andout via upper fitting 210 and sample line 126. In order to minimizeneutron activation of the filter element, the element is preferablyfabricated of materials which are not readily activated, or have veryshort half-lived radioactive daughter products. Such materials aszirconium and aluminum, and certain of their alloys, and the like andwhich do not contain readily activatable impurities, such as cobalt, forexample, are suitable. Further, holder tube 224 is preferably fabricatedof a corrosion-resistant material which also acts as a neutron shield,such as boron-aluminum alloys, boron steels, cadmium-containing mixturesand alloys, and the like. The sample lines are utilized in thisembodiment to carry sample filtrate to a common header or manifoldmaintained at a reduced pressure, and the sampling llow rate iscontrolled in the manner as shown in FIGURE 3.

Referring now to FIGURES l and 11, two additional types of filterelements are shown which may be employed in the present invention.

In FIGURE 10 sample line 126 is extended coaxially through holder vessel23R) provided with removable cap 232 and an annular type filter element234. Sample line 126 is capped at its upper end at 236 and is providedwith perforations 238 within element 234. Vent line 240 opens into ventmanifold 242 in which the controlled low pressure is maintained. Thecaps 232 and 236 are removable permitting removal, monitoring, orreplacement of filter element 234. Suitable filter materials includetightly packed wools of stainless steel, glass, graphite, or nickel andthe like.

In FIGURE 11 sample line 12.6 with perforations 253 opens into a holdervessel 230 and is capped at 236. Also vent line 240 opens to a ventmanifold 242 common to all such filter holders. Projecting radiallyoutward from sample line 126 and inwardly from holder wall are supportshelves 244 and 246 respectively on which is supported an annular-shapedfilter element disc 243. The outer edge of disc 248 is turned up at 250to permit stacking and automatic monitoring of a plurality of such discsin commercially available automatic monitoring devices. The sample fiowsthrough line 126 and perforations 23 3 downwardly through the disc andthe filtrate is vented through line 240 and manifold 242. The disc'maybe a permeable ceramic or sintered powder of metal or other material.

Other types of filter elements can, from the foregoing disclosure, beadapted readily as embodiments of this invention by those skilled in theart.

Referring now to FIGURES 12 and 13, a cross section elevation and a planview of another embodiment of this invention are illustratedrespectively. This utilizes the flow of coolant in the fuel channel togenerate the necessary pressure differentials required to filter asample stream and requires no vent line opening through the reactorvessel wall. It is particularly well suited to natural circulation,boiling-liquid cooled reactors. In the drawings, fuel channel 260 isprovided with an extension or chimney 262 which extends above the top264 of the fuel or active zone. A streamlined filter holder 266 issupported centrally or otherwise in chimney 252 thereby forming aperipheral restricted or reduced cross sectional area 268 in the chimneyopen to coolant flow. Disposed coaxially in holder 266 and extendingupwardly from a lower opening 270 is inlet tube 272 also open at itsupper end 274 adjacent the other end of the holder. Disposed around thelargest perimeter of holder 266 are several openings 276 which aredisposed at longitudinal position between the ends of tube 272. Filterelement 278 is disposed within holder 266 between holder openings 276and tube openings 274. The device is supported from grid 2% by means ofsupport element 232.

In operation, the coolant flows upwardly past holder 256, throughrestricted area 268, and through grid 280, at some average operatingpressure P,,. The inlet opening 27%) receives a small sample of thecurrent of coolant at a pressure P, which is equal to or slightly higherthan P The restricted area 268 constitutes the throat of an annularVenturi in which a substantially increased fluid velocity is generated,with a correspondingly reduced fluid pressure P P being less than P Thedifferential pressure P,-P is the driving force which moves the smallcoolant sample up through inlet tube 272 and through filter element 278leaving on the element any small particulate solids escaping from adefective fuel element in the channel. Other modifications of thisembodiment will occur to those skilled in the art based upon the Pitotand Venturi principles applied here. A suitable filter element is anannular-shaped plate of permeable ceramic or refractory such as aluminumoxide, or the permeable sintered metal materials referred to above.

Referring finally to FIGURE 14, an elevation view of a shieldedscintillation spectrophotometer is shown applied to the measurment ofthe activity of filter elements of the type shown in FIGURES l2 and 13,but which is equally applicable to the monitoring of any of the otherfilter elements and whether in the grid, as in FIGURE 14, or in theholder vessel as in FIGURE 3, or individually. In FIGURE 14 fuel channelchimneys 262, filter element holder 266, filter element 278, grid 280,and support element 232 are shown as in FIGURE 12. The grid and .lterelement holder assembly may be removed from above the reactor core, orthe monitor instrument may be introduced above the grid as indicated inFIGURE 14. This particular monitor consists of heavy shield 29% of leadprovided with an inner space 292 containing scintillation crystal 294such as sodium iodide, disposed adjacent a photomultiplier tube Zfid.These are well known, commercially available components which operate inconnection with electronic amplification, indicating, and recordingequipment, also well known to those skilled in the art. The lowerportion of shield 295i is provided with tip portion 2% provided with aplurality of downwardly converging openings 3tlil aligned with focalpoint 3&2, any convenient point on or near the filter element 278. Onlyradiation generated by radioactive fuel or fission'product particles onfilter element 278 in which focus 392 is located can reach the crystal2%. This permits the removal of the filter elements with the samplinggrid and individual monitoring to be done without dismantling orreplacing the individual holders. However, filters indicating highactivity should be replaced along with the corresponding defectivechannel. Further, scintillation spectrometers are capable of being maderesponsive to alpha, beta, gamma, and neutron radiation having aparticular energy spectrum which may be characteristic of a particularfission product or activated fuel component. It

- thus may be adjusted to respond to Ba for example,

and reject or be insensitive to radiation from various activatedcorrosion or erosion products, or activated coolant. This is a highlypreferred system for detecting 1 1 and measuring the presence ofparticulate matter on the filter elements according to this invention.

Other known means for detection and measurement of the various kinds ofradiation from radioactive solids may be used in conjunction with thisinvention, including for example, gamma and beta radiation spectroscopy,mass spectrometry, flame and are spectroscopy, fluophometry, and others.Further, various chemical treatments of the filter may be used insteadof or in conjunction with these various radiation detection systems. Forexample, in a reactor system where the structural corrosion and erosionproduct may be confused for the selected fission product activity, thematerial on the filter may be completely dissolved in an acid and thedesired fission products separated by precipitation. For example, B21can be separated from the solution as barium sulphate.

A particular embodiment of this invention has been described inconsiderable detail by way of illustration. It should be understood thatvarious other modifications and adaptations thereof may be made by thoseskilled in this particular art without departing from the spirit andscope of this invention as set forth in the following claims.

I claim:

1. An apparatus for accumulating a sample of particulate matter releasedfrom a defective nuclear reactor fuel assembly containing a leaking fuelelement, which apparatus comprises a holder vessel disposed at thecoolant outlet end of said fuel assembly forming a peripheral region ofreduced coolant flow area in said outlet around said holder vessel, afilter element having inlet and outlet portions and disposed within saidvessel, said vessel having an inlet opening communicating with the inletportion of said filter element and disposed in coolant-receivingrelation to the fuel assembly upstream relative to the coolant flowdirection from the region of reduced flow area, said vessel furtherhaving at least one outlet opening communicating with the outlet portionof said filter element and disposed substantially at said region ofreduced flow area.

2. An apparatus for accumulating a sample of particulate matter releasedfrom a defective nuclear reactor fuel assembly containing a leaking fuelelement, which assembly is contained within a tubular coolant flowchannel, which apparatus comprises a holder vessel disposed in theoutlet end of said coolant flow channel and forming between theperiphery of said vessel and said channel a peripheral region of reducedcoolant flow area, a filter element having inlet and outlet portions anddisposed within said vessel, said vessel having an inlet openingcommunicating with the inlet portion of said filter and disposedupstream relative to the coolant flow direction from the region ofreduced coolant flow area, said vessel further having at least oneoutlet opening communicating with the outlet portion of said filterelement and disposed at the region of reduced flow area.

3. An apparatus for accumulating a sample of particulate matter releasedfrom a defective nuclear reactor fuel assembly containing a leaking fuelelement, which assembly is contained within a tubular coolant flowchannel, which apparatus comprises a streamlined holder vessel disposedin the outlet end of said coolant flow channel thereby forming aperipheral region of reduced coolant flow area, a filter element havinginlet and outlet positions and disposed within said vessel, the inletportion of said filter communicating through an inlet opening in saidholder vessel with said coolant flow channel at a point upstreamrelative to the coolant flow direction from said region of reduced flowarea, the outlet portion of said filter communicating through at leastone outlet opening in said holder vessel with said coolant flow channelat said region of reduced flow area, whereby coolant flow through saidflow channel past said holder vessel induces a flow of a sample of saidcoolant fluid through said filter element by Pitot and Venturi forces toaccumulate on 12 said filter element particulate matter released intosaid coolant from said leaking fuel element.

4. An apparatus according to claim 3 wherein said holder vessel isprovided with a plurality of outlet openings disposed around said vesselat its largest perimeter at said region of reduced flow area.

5. An apparatus according to claim 3 wherein said filter element isdisposed within said holder vessel at a point downstream relative to thecoolant flow direction from said outlet opening, in combination withconduit means connecting said inlet opening with the region within saidholder vessel downstream relative to the coolant flow direction fromsaid filter element.

6. An apparatus for accumulating a sample of particulate matter releasedfrom a flow channel containing a leaking fuel element in a nuclearreactor which contains a plurality of flow channels through which afluid is passed, which apparatus comprises a holder vessel containing afilter element having inlet and outlet ends and disposed in each of saidflow channels to restrict the area thereof open to fluid flow at a pointadjacent the outlet ends of said channels, said vessel being providedwith an inlet communicating with the inlet end of said filter elementand in direct fluid receiving relation to the unrestricted part of saidflow channel upstream from said holder vessel relative to the coolantflow direction, said vessel being further provided with an outletcommunicating with the outlet end of said filter and with the restrictedflow area of said channel adjacent said holder vessel whereby fluid flowtherethrough induces a flow of a sample of said fluid through saidfilter element.

7. An apparatus for accumulating a sample of particulate matter releasedfrom a flow channel containing a leaking fuel element in a nuclearreactor which contains a plurality of flow channels having inlet andoutlet ends and through which a fluid is passed, which apparatuscomprises a holder vessel containing a filter element having inlet andoutlet ends and disposed in each of said flow channels to provide aperipheral region therein of reduced area open to fluid flow at a pointadjacent the outlet ends of said channels, said vessel being providedwith an inlet opening communicating with the inlet end of said filterelement and in direct fluid receiving relation to an unrestricted partof said' flow channel upstream from said holder vessel relative to thecoolant, flow direction, said vessel being further provided with atleast one outlet opening communicating with the outlet end of saidfilter and with said peripheral region of reduced flow area of saidchannel, whereby fluid flow therethrough induces a flow of a sample ofsaid fluid through said filter element by Pitot and Venturi forces toaccumulate on said filter element particulate matter released into saidfluid from said leaking fuel element.

8. In a nuclear reactor apparatus comprising a pressure vessel, anuclear chain reacting core disposed within said pressure vessel andhaving a plurality of coolant flow channels having inlet and outlet endsand containing nuclear fuel elements, and means for passing a coolantfluid through said channels to remove heat therefrom, the improvedapparatus for accumulating a sample of particulate matter released intoa flow channel containing a leaking fuel element which comprises asupporting structure disposed immediately adjacent the coolant outletend of said core, a plurality of holder vessels supported by saidstructure, a holder vessel being thereby disposed in each of said flowchannels to restrict the area thereof open to fluid flow at a pointadjacent the outlet ends of said channels, a solid fluid-permeablefilter element having inlet and outlet ends and disposed in each holdervessel, each such holder vessel being provided with an inletcommunicating with the inlet end of said filter element and in directfluid receiving relation to a fuel channel at a point upstream from thepoint of the restricted area around said holder vessel relative to thecoolant flow direction, each such holder vessel being 13 furtherprovided with an outlet communicating with the outlet end of said filterand with the restricted flow area of said channel around said holdervessel whereby fluid flow therethrough induces a flow of a sample ofsaid fluid through said filter element.

9. In a nuclear reactor apparatus comprising a pressure vessel, anuclear chain reacting core disposed within said pressure vessel andhaving a plurality of coolant flow channels having inlet and outlet endsand containing nuclear fuel elements, and means for passing a coolantfluid through said channels to remove heat therefrom, the improvedapparatus for accumulating a sample of particulate matter released intoa flow channel containing a leaking fuel element which comprises asupporting structure disposed immediately adjacent the effluent end ofsaid core, a plurality of holder vessels supported from said supportingstructure and disposed one each in the outlet end of each of saidcoolant fiow channels to provide a peripheral region in each channel ofreduced open coolant flow area, a solid fluid-permeable filter elementhaving inlet and outlet ends and disposed in each holder vessel, each ofsaid holder vessels being provided with an inlet opening comunicatingwith the inlet end of said filter element and in direct fluid receivingrelation to said flow channel at a point upstream relative to thecoolant flow direction from said region of reduced open coolant flowarea, said holder vessels being each further provided with a pluralityof outlet openings communicating with the outlet end of said filterelement and with said peripheral region of reduced open coolant flowarea, whereby coolant fluid flow through each channel induces a sampleflow of coolant fluid by Pitot and Venturi forces through the filterelement disposed in said channel to accumulate on said filter elementradioactive particulate matter released into said coolant from a leakingfuel element disposed within said channel.

OTHER REFERENCES Labeyrie et al.: 1955 Geneva Conf. on Peaceful Uses ofAtomic Energy, vol. 3, pp. 86-90, publ. by UN.

CARL D. QUARFORTH, Primary Examiner.

1. AN APPARATUS FOR ACCUMULATING A SAMPLE OF PARTICULATE MATTER RELEASED FROM A DEFECTIVE NUCLEAR REACTOR FUEL ASSEMBLY CONTAINING A LEAKING FUEL ELEMENT, WHICH APPARATUS COMPRISES A HOLDER VESSEL DISPOSED AT THE COOLANT OUTLET END OF SAID FUEL ASSEMBLY FORMING A PERIPHERAL REGION OF REDUCED COOLANT FLOW AREA IN SAID OUTLET AROUND SAID HOLDER VESSEL, A FILTER ELEMENT HAVING INLET AND OUTLET PORTIONS AND DISPOSED WITHIN SAID VESSEL, SAID VESSEL HAVING AN INLET OPENING COMMUNICATING WITH THE INLET PORTION OF SAID FILTER ELEMENT AND DISPOSED IN COOLANT-RECEIVING RELATION TO THE FUEL ASSEMBLY UPSTREAM RELATIVE TO THE COOLANT FLOW DIRECTION FROM THE REGION OF REDUCED FLOW AREA, SAID VESSEL FURTHER HAVING AT LEAST ONE OUTLET OPENING COMMUNICATING WITH THE OUTLET PORTION OF SAID FILTER ELEMENT AND DISPOSED SUBSTANTIALLY AT SAID REGION OF REDUCED FLOW AREA. 